Various principles are known for recording information in a thin film (recording film) by radiating a laser beam on it. Among them, a method utilizing the change of the atomic arrangement by the radiation of the laser beam such as the phase transition (also called the phase change) of the film material or the photo darkening is not substantially accompanied by the deformation of the thin film, and therefore has the advantage that an information recording medium of a two-side disk structure can be obtained by attaching two disk members directly to each other.
Generally, these information recording media are configured of a substrate, and a protective layer, a recording film of GeSbTe group, etc., a protective layer and a reflective layer formed on the substrate, and the reflectance is higher in crystal state than in amorphous state. Therefore, the absorption of a recording film is larger for amorphus. The recording mark portion in amorphous state is liable to increase in temperature more easily than the crystal. In the case where an overwrite operation is performed in this state, therefore, a newly recorded mark is increased excessively, thereby causing the reproduction signal to be distorted.
In order to prevent this inconvenience, an effort has been made to increase the absorption coefficient of the recording film in crystal state as compared with that of the recording film in amorphous state. For example, reference 1 "Yamada and three others, Shingaku Giho MR92-71, CPM92-148, December 1992, p.37" describes a structure formed with an Au reflective layer as thin as 10 nm to reverse the absorption coefficient.
Also, reference 2 "Okada and six others, Shingaku Giho MR93-53, CPM93-105, December 1993, p. 1" describes a structure in which Si of 65 nm is used for the reflective layer thereby to reverse the absorption coefficient.
In this specification, the term "phase change" is defined to include not only the phase change between crystal and amorphus but also between melting (change to liquid phase) and recrystallization and between crystal states.